The present invention generally relates to a pattern checking method and a pattern checking apparatus for checking an image of a pattern to be checked (a checking pattern) and also a method and an apparatus for checking a defect of a checking pattern by utilizing the checking method/apparatus. More specifically, the present invention is directed to a pattern checking method and a pattern checking apparatus suitable for checking an outer appearance of a pattern such as a semiconductor wafer and a liquid crystal display.
Conventionally, this sort of checking apparatus is known in the art, for instance, as described in the Japanese publication of JP-A-59-192943 (1984). In this publication, while a plurality of checking patterns which are successively arranged, are moved at a preset velocity, the images of these checking patterns are sequentially detected by means of imaging elements, e.g., a line sensor. The positional shift between the detected image signal and the image signal which has been delayed by a time period equal to a difference in the detecting times of two checking patterns, is corrected at predetermined time periods. After these image signals are made to be coincident with each other at high precision, these signals are compared, whereby a discrepant point between these image signals is recognized as a defect. This positional-shift correction is performed in such a manner that, for instance, the pattern edges of the respective checking patterns are detected to detect these positional shifts, and then the delay amount of the delayed image signal is controlled in response to this detected amount.
As the line sensor, very recently, either a one-dimensional CCD (charge-coupled-device) line sensor, or a TDI (time delay and integration) CCD image sensor has been used to detect an image of a pattern of interest with employment of an objective lens having a relatively large magnification. It should be noted that a TDI image sensor is such an image sensor having a structure that a plurality of one-dimensional image sensors are arranged in a two-dimensional form, and in which the output signals from the respective one-dimensional image sensors are delayed by preselected delay times, and the output signals from the one-dimensional image sensors which are positioned adjacent to each other and scan the same position of the object are added to each other, whereby the detecting light amount is increased.
However, there are problems with such a sensor structure being that the respective layers of the multi-layer as the object are overlapped with each other, thereby causing the multi-layer to have increasingly higher stepped portions and concave/convex portions. Also an objective lens having a great magnification has a short depth of focus and therefore only the patterns focused onto the image sensors in a narrow range may be detected as pattern images. Further other major portions of the patterns are blurred. As a result, only a portion of the pattern which can be focused is checked in accordance with the conventional checking method. There are other problems being that the defect detecting sensitivities with respect to the remaining major pattern portions which are out of focus become low, and thus a pattern checking operation with high reliability cannot be realized.
In accordance with the conventional checking methods and apparatuses, since the above-described positional shift corrections performed every predetermined time period are separately executed for the respective images, namely within a relatively narrow area, there are some risks that the pattern edges cannot be detected at the places whose pattern density is low, or positioned near a significant/large pattern defect. Also, since such a positional shift correction may be performed at a defective edge, correct positioning operations can not be performed nor expected. As a consequence, an erroneous detection wherein a normal pattern part is mistakenly detected as a pattern defect may occur as a result of use of the above-described conventional techniques. Thus, a checking operation with high reliability cannot be realized using the above-described conventional technique.
Since it is difficult in practice to continuously move the checking patterns at a constant speed, there are problems that image distortions may be caused by speed variations, or vibrations of the detecting optical system. Therefore, since the positional shifts are produced due to many factors, the unit of the positional shift correction, namely the area of the image must be designed to a suitable value in order not to receive such adverse influences. However, it is difficult to optimize an aperiodic distortion, or a performance specific to the checking apparatus. Accordingly, no correct positioning operation can be performed, and erroneous detecting operation may occur wherein a normal pattern part may be detected as a defect.